Use snake cased GraphQL argument

Publishing API has switched to use snake cased fields and arguments for
GraphQL queries in alphagov/publishing-api#3049.

This updates the pact test to reflect this.

test/pacts/publishing_api/graphql_query_pact_test.rb

@@ -9,7 +9,7 @@
   it "returns the response to the query" do
     query = <<~QUERY
       {
-        edition(basePath: "/my-document") {
+        edition(base_path: "/my-document") {
           ... on Edition {
             title
           }